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Abstract

In this article a new method is presented that allows for low loss implementation of fast carrier transport structures in diffraction limited photonic crystal resonators. We utilize a ‘node-matched doping’ process in which precise silicon doping results in comb-like shaped, highly-doped diode areas that are matched to the spatial field distribution of the optical modes of a Fabry-Pérot resonator. While the doping is only applied to areas with low optical field strength, the intrinsic diode region overlaps with an optical field maximum. The presented node-matched diode-modulators, combining small size, high-speed, thermal stability and energy-efficient switching could become the centerpiece for monolithically integrated transceivers.

Figures (5)

Fig. 1 Illustration of the node-matched diode principle and fabrication details. a, Fabry-Pérot resonator including the tapered photonic crystal structure serving as mirrors. The colour coded ellipses represent the periodic anti-nodes of the electric field strength. Comb-shaped areas in blue and red are the p+- and n+-doped regions of the p-i-n-diode, respectively, intersecting the waveguide at the locations of the resonator nodes. The arrows on the left and right side indicate the cw input and modulated output radiation. b, Simulation of the node matched diode operation principle. The injected carriers lead to strong absorption and therefore field strength reduction in the resonator anti-nodes. c, A vertical cross-section of a real device, recorded by a focused ion beam system. d, Corresponding schematic of the structural assembly with the intersecting plane located at the position of an n+-doped comb segment. The blue striped area indicates the p+-doped area, which is displaced by λ/(2·neff) = 295 nm along the waveguide propagation axis. The dashed black line inside the waveguide denotes the hole dimensions of the photonic crystal.

Fig. 2 Schematic of the experimental setup. Block I shows the preparation of the polarized cw light source. Block II and Block III depict the generation of the electrical driver signals. In Block IV, the temperature controlled DUT and the light coupling are illustrated. Spectral and temporal detection systems are represented by Blocks V and VI, respectively.

Fig. 3 Simulated distribution of the optical field intensity and voltage dependent spectral properties of the NMD-modulator. a, Optical field distribution in the NMD resonator. The waveguide including the photonic crystal mirrors and the node-matched comb diode segments are plotted to scale. b, Transversal field distribution of the guided optical mode. The waveguide dimensions are indicated by the white shape. c, Voltage dependence of the wavelength shift Δλ due to the plasma dispersion effect in silicon. The left side scale measures the corresponding change of the refractive index Δn = neff ·Δλ/λpeak, with neff = 2.63 being the effective refractive index and λpeak the peak wavelength. d, The grey line depicts the passive unmodulated transmission spectrum around the peak at 1540 nm with a FWHM of 6.3 nm, corresponding to an unloaded resonator Q-factor of 244. The black line represents the transmission spectrum temporally integrated over the two states with and without applied driver voltage. 10 dB modulation contrast is available at 1546 nm modulation wavelength. The driver high-state transmission around the modulation wavelength is illustrated by the red dotted line assuming an identical but shifted progression as for the low-state transmission peak.

Fig. 4 Small-signal low-pass filter characteristics and comparison of eye diagrams from modulators with a homogeneous diode and an NMD. a, Cut-off frequencies and attenuation slopes of two identical resonators with a homogeneous diode (black) and an NMD (red) have been determined from their filter characteristics at 0.2 Vpp with 1.1 Vbias, respectively. b,c, Eye diagrams (inverted) from both modulator types at the same bandwidth of 12.5 GBaud are shown after electrical amplification.

Fig. 5 Rise and fall times, τr and τf, and eye diagrams of an NMD-modulator with a 35 μm long FP-resonator structure and 30 μm cavity length. a, Transition times at different forward bias voltage without additional reverse bias voltage. A rise time reduction from 555 ps to 160 ps was observed for increasing the Ufb from 0.8 V to 2.2 V, respectively while the fall times increase by a factor of two. b, Transition times at different reverse bias voltage and constant Ufb of 1.4 V. The additionally applied reverse bias voltage leads as well to a rise time decrease by more than a factor of three from 436 ps down to 141 ps for Urb between 0 V and 3.6 V, respectively. A Urb of 3.6 V applied to the NMD-modulator lowers the fall time down to 46 ps. c, The inset depicts a modulated signal trace at 400 MBaud bandwidth with the measured 90/10 levels. d, Open eyes (inverted) with clearly separated 1 and 0 states were generated at 12.5 GBaud modulation bandwidth after electrical amplification. e, Eye diagrams (inverted) with high signal contrast have been achieved even at a bandwidth of 25 GBaud.